/*
 * Copyright 2022 Deep Learning on Flink Authors
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.flinkextended.flink.ml.tensorflow.data;

import org.apache.hadoop.util.PureJavaCrc32C;

import java.util.zip.Checksum;

/** A pure-java implementation of the CRC32 checksum. */
public class Crc32C implements Checksum {
    private static final int MASK_DELTA = 0xa282ead8;
    private final PureJavaCrc32C crc32C;

    public static int maskedCrc32c(byte[] data) {
        return maskedCrc32c(data, 0, data.length);
    }

    public static int maskedCrc32c(byte[] data, int offset, int length) {
        Crc32C crc32c = new Crc32C();
        crc32c.update(data, offset, length);
        return crc32c.getMaskedValue();
    }

    /**
     * Return a masked representation of crc.
     *
     * <p>Motivation: it is problematic to compute the CRC of a string that contains embedded CRCs.
     * Therefore we recommend that CRCs stored somewhere (e.g., in files) should be masked before
     * being stored.
     *
     * @param crc CRC
     * @return masked CRC
     */
    public static int mask(int crc) {
        // Rotate right by 15 bits and add a constant.
        return ((crc >>> 15) | (crc << 17)) + MASK_DELTA;
    }

    /**
     * Return the crc whose masked representation is masked_crc.
     *
     * @param maskedCrc masked CRC
     * @return crc whose masked representation is masked_crc
     */
    public static int unmask(int maskedCrc) {
        int rot = maskedCrc - MASK_DELTA;
        return ((rot >>> 17) | (rot << 15));
    }

    public Crc32C() {
        crc32C = new PureJavaCrc32C();
    }

    public int getMaskedValue() {
        return mask(getIntValue());
    }

    public int getIntValue() {
        return (int) getValue();
    }

    @Override
    public void update(int b) {
        crc32C.update(b);
    }

    @Override
    public void update(byte[] b, int off, int len) {
        crc32C.update(b, off, len);
    }

    @Override
    public long getValue() {
        return crc32C.getValue();
    }

    @Override
    public void reset() {
        crc32C.reset();
    }
}
